Hurricane Harvey's Size And Impact Points To Climate ChangeHurricane Harvey bears the marks of climate change. Warmer waters in the Gulf of Mexico and rising sea levels make rain heavy storms like Harvey more likely in the future.

Hurricane Harvey bears the marks of climate change. Warmer waters in the Gulf of Mexico and rising sea levels make rain heavy storms like Harvey more likely in the future.

KELLY MCEVERS, HOST:

Many people are wondering whether climate change has anything to do with Harvey's size and impact. NPR's science correspondent Christopher Joyce covers climate issues, and he is with us now. Hi, there.

CHRISTOPHER JOYCE, BYLINE: Hi, Kelly.

MCEVERS: So one of the terms that the National Weather Service keeps using for Harvey is unprecedented. And the question is, is that because of climate change?

JOYCE: Can I give you a yes and a no on that?

MCEVERS: Sure.

JOYCE: OK.

MCEVERS: (Laughter) Start with the yes.

JOYCE: All right - with the yes. I mean as you know, there have always been big storms. But climate change is causing some other things to change. And most important here is the ocean. It's warming up. And when you heat up - heat is energy, and so if there's more heat, there's more energy for storms to feed off of. And so the world's oceans already on average have warmed up for well over a degree over the past century.

And in addition, the Gulf of Mexico has been crazy the last few weeks. I mean it's been two or three degrees more - warmer than normal. So you add it altogether; it's been four degrees warmer than normal in the Gulf. And that's a lot of energy. And the other effect of heat - it causes ocean water to evaporate and rise up. So with every extra degree in ocean temperature you add, that produces a heck of a lot of water vapor.

MCEVERS: I guess once that moisture goes up in a storm like this, it has to come back down. Is that why we're seeing all the rain all over Texas?

JOYCE: Right. In fact, you know, you can see from the satellite parts of the storm system were observed to be extending out from the main body back kind of like a tentacle - back over the Gulf and reaching back and picking up more heat and more water vapor and then sucking it back onto land.

MCEVERS: What about sea level rise? Has that had a role in the storm?

JOYCE: Well, warming seas do expand, and they rise. And you add melting ice and snow, and you get sea level rise. And it has - the world's oceans are about 6 inches, 7 inches higher than they were a century ago. And that increases storm surge. That was a big deal with Sandy, for example, in 2012. But with Harvey so far, it's really more a matter of this incredible rain.

MCEVERS: OK, so climate change appears to have made Harvey worse than it would have been otherwise. But what are some of the other factors? What's the no here?

JOYCE: Well, one of the reasons is that it stalled. It's - it really took its time. At one point, the storm was moving about as fast as you can walk. So it's dumping area - dumping water in one place instead of moving elsewhere. And so there's no clear link to climate change on that.

Also, Houston and the whole area's seen an enormous amount of development. There's more impermeable surfaces, more shopping malls, more highways. And you've probably been to Houston. You know it's flat, you know? There's nowhere for the water to go.

MCEVERS: Scientists do predict that the climate will continue to warm. So how will that change things in the future with future storms?

JOYCE: Well, scientists are pretty low to predict too specifically about what happens. Sea level rise is fairly easy to measure. I mean you can predict pretty well what a storm surge is going to be along the coastline for a given storm if you know sea level rise. As for the storms themselves, if the oceans get hotter, you know, a really big storm that might happen once every hundred years now may happen every 50 or every 20. And that may actually be happening already, but you can't tell where and when.

A scientist once told me about climate and weather, said it's kind of like playing baseball with a bat with lead in it. You know, you're going to go out there, and you're going to hit foul balls, and you're going to hit grounders, and you're going to strike out. But every once in a while, you hit that sweet spot where that leaded bat, and it's not just going to knock the ball into the stands. It's going to knock it out of the park.

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